9-[(R)-2-[[(R,S)-[[(S)-1-(Isopropoxycarbonyl)ethyl]amino]-phenoxyphosphinyl]methoxy]propyl]adenine, an isopropylalaninyl monoamidate phenyl monoester prodrug of tenofovir, is a diastereomeric mixture. It was first disclosed in PCT Patent Application, Publication Number WO200208241. The process for separation of the diastereomers of 9-[(R)-2-[[(R,S)-[[(S)-1-(Isopropoxycarbonyl)ethyl]amino]-phenoxyphosphinyl]methoxy]propyl]adenine disclosed in WO200208241 involves the use of chromatography or crystallization techniques. The chromatography techniques described are batch elution chromatography, simulated bed chromatography and C18 Reverse phase high performance liquid chromatography.